1. Field of the Invention
This invention relates to a process for improved methane production by two-phase anaerobic digestion of organic carbonaceous materials. More particularly, this invention relates to an improved two-phase anaerobic digestion process for methane production wherein conditions are provided to efficiently conduct an acid forming phase and to separately conduct a methane production phase. More particularly, this invention relates to an anaerobic digestion process for producing methane from a wide variety of biomass and/or solid organic wastes.
2. Description of Prior Art
Anaerobic digestion is known for stabilizing sludge and predominately organic materials and for producing usable product gas of varying composition. The organic feed mixture that provides the substrate for anaerobic biodegradation may comprise a wide variety of organic carbon sources, ranging from raw sewage, sludge to municipal refuse, or biomass material such as plants and crop waste. The process of anaerobic digestion biodegrades any of these organic carbonaceous materials, under appropriate operating conditions, to form product gas that contains desirable methane gas. A typical anaerobic digester product gas may contain, on a dry basis, 55% to 65% methane, 35% to 45% carbon dioxide, and less than 1% of other gases.
Current practice of digesting solid organic wastes in landfills is inefficient while anaerobic digestion of solid organic wastes in a tank-based system requires extensive front-end processing and careful classification of the waste to be processed. In addition, upgrading the gas mixture to a pipeline quality gas, that is, a gas having greater than 95% methane, using known unit processes for carbon dioxide, hydrogen sulfide, and moisture removal, are expensive and may exceed the digestion costs.
Separated two-phase anaerobic digestion processes, where the acid phase digestion and the methane phase digestion are carried out in two separate reactor vessels, have been found to enhance the efficiency of conversion of organic carbonaceous materials to methane, such as described in Pohland and Ghosh, Bio-Technology and Bio-Engineering Symposium No. 2,85-106 (1971) John Wiley and Sons, Inc., and by the same authors in Environmental Letters, 1(4), 255-266 (1971), Marcel Dekker, Inc. In an acid first phase, the microbial population and operating conditions are selected to promote the conversion of organic carbonaceous matter to volatile fatty acids of low molecular weight. The volatile fatty acids remain solubilized in the liquid portion of the digester contents. The liquid/solid effluent from the acid first phase is conveyed to a methane second phase, in which methanogenic microorganisms convert the volatile fatty acids to product gas composed primarily of methane and carbon dioxide. Product gas is removed from the methane second phase and processed or scrubbed to separate the methane component that is drawn off as pipeline gas. As previously stated, the gas separation of methane is an expensive process that detracts from the economic feasibility of the anaerobic biodegradation of organic carbonaceous material to produce methane gas.
Two-phase anaerobic digestion of organic carbonaceous materials to produce methane is generally taught by U.S. Pat. No. 4,022,665, U.S. Pat. No. 4,318,993, and U.S. Pat. No. 4,696,746. Each of these patents teaches the conduct of acid phase digestion and methane phase digestion in two separate reactor vessels. Each of these patents also teaches operating conditions for acid phase and methane phase digestion. The improved process of the present invention employs the operating conditions for a two-phase anaerobic digestion process, such as feed rates and detention times, taught by these patents and, thus, the teachings of these patents with respect to such operating conditions are incorporated herein by reference.
Two-phase anaerobic digestion can be carried out in a single digestion vessel as taught, for example, by U.S. Pat. No. 4,735,724 which teaches a non-mixed vertical tower anaerobic digester and anaerobic digestion process which provides passive concentration of biodegradable feed solids and microorganisms in an upper portion of a continuous digester volume and effluent withdrawal from the middle to the bottom portion of the digester, resulting in increased solids retention times, reduced hydraulic retention times, and enhanced bio-conversion efficiency.
To enhance the methane content of product gas derived from two-phase anaerobic digestion of organic carbonaceous materials, U.S. Pat. No. 4,722,741 teaches the removal of a large portion of carbon dioxide contaminate from the acid forming phase and from the liquid product of the acid forming phase, and by absorption of carbon dioxide into the liquid phase during the methane forming phase of the anaerobic digestion process. Chynoweth, D. P. et al., "Biological Assessments of Anaerobic Digestion of Municipal Solid Waste", final report submitted to the Solar Energy Research Institute under U.S. DOE Contract No. DE-AC02-83CH10093 teaches the use of air stripping of recirculated digester supernatant to enrich methane content of biogas produced in a combined acid phase/methane phase digester. This report also teaches that as the stripper aeration rate is gradually increased, the level of digester product gas methane concentration increases. The report further indicates that a biological impact assessment of the disclosed process indicates that the addition of air to the anaerobic digestion process does not have a negative impact on the digestion process, and, in fact, stimulates the ability of the overall digester microbial population to form methane. However, aeration of digester supernatant, for removal of carbon dioxide, was anticipated to have a potential influence on microbial activity within the digester system, resulting from shock to anaerobic bacteria during the aeration step, or to transport of oxygen back to the digester during supernatant recycle. Thus, in order to prevent the latter, aerated supernatant was retained in a holding tank to allow facultative bacteria to reduce dissolved oxygen levels. Accordingly, this report suggests that the presence of oxygen in a combined two phase-anaerobic digestion vessel may have negative impact with respect to the production of methane. Indeed, it is known that exposure of methanogenic microorganisms to oxygen kills the microorganisms.